Early Dark Energy beyond slow-roll: implications for cosmic tensions

TL;DR

This paper investigates dynamical Early Dark Energy with variable equation of state parameters, finding mild data preferences for non-constant EDE but concluding it cannot fully resolve the H0 and sigma8 cosmological tensions.

Contribution

It introduces a parameterization allowing for a dynamical EDE with variable initial and final equations of state, exploring its effects on cosmological tensions.

Findings

Mild preference for non-constant EDE with w_i=-0.78

Allowing w_i≠-1 slightly reduces sigma8

Dynamical EDE cannot fully resolve H0 and sigma8 tensions

Abstract

In this work, we explore the possibility that Early Dark Energy (EDE) is dynamical in nature and study its effect on cosmological observables. We introduce a parameterization of the equation of state allowing for an equation of state $w$ differing considerably from cosmological constant (cc, $w={-1}$) and vary both the initial $w_i$ as well final $w_f$ equation of state of the EDE fluid. This idea is motivated by the fact that in many models of EDE, the scalar field may have some kinetic energy when it starts to behave like EDE before the CMB decoupling. We find that the present data have a mild preference for non-cc early dark energy $( w_i= -0.78)$ using Planck+BAO+Pantheon+S$H_0$ES data sets, leading to $\Delta \chi^2_{\rm min}$ improvement of -2.5 at the expense of one more parameter. However, $w_i$ is only weakly constrained, with $w_i < -0.56$ at $1\sigma$. We argue that allowing for $w_i\neq -1$ can play a role in decreasing the $\sigma_8$ parameter. Yet, in practice the decrease is only $\sim0.4\sigma$ and $\sigma_8$ is still larger than weak lensing measurements. We conclude that while promising, a dynamical EDE cannot resolve both $H_0$ and $\sigma_8$ tensions simultaneously.